Disk drives are well known in the computer art for providing secondary mass storage with random access. A disk drive essentially comprises one or more magnetic data storage disks rotating on a spindle by a spindle motor, within an enclosed housing. A magnetic transducer head is placed on an actuator arm system and positioned very closely to each data storage surface by a slider suspended upon an air bearing. Servo information are typically written in servo sectors which interrupt data sectors or blocks. Servo information provide a servo control loop in the disk drive with head position information to enable a head positioner mechanism, such as a rotary voice coil motor, to move the actuator, and therefore the head, from track to track during random access track seeking operations, and to maintain the head in proper alignment with a track centerline during track following operations when user data is written to or read from the available data block storage areas of the disk surface.
A such, the servo control loop is used to control head positioning as the head is being moved transversely across tracks by the actuator, and to cause the head to remain over a particular data track as the disk spins. The servo loop controls the acceleration of the head which results from a force supplied by the electric motor on the actuator.
Servo-controlled actuator systems experience serious problems due to mechanical actuator resonances. As described in U.S. Pat. No. 6,064,540, by Huang, et al., these vibrational modes include the natural modes of the actuator and those of any intervening mechanical components. With increasing mechanical complexity, the vibrational modes of any given actuator system become difficult to predict. The problem is further compounded as the operating frequency of the actuator system is increased. The vibrational modes limit the control loop gain of the servo system, reduce bandwidth of the servo system, or both. This causes the controlled element, such as a transducer head, to experience excessive settling time after positioning, poor response to disturbances, poor tracking ability, or any combination of these.
Prior art systems have attempted to ensure stable operation of actuator systems by stabilizing the control loop. This has been done by inserting gain stabilizing filters such as electronic notch filters in the control loop path. These filters are placed in the downstream portion of the control loop to filter out the signal information within the band reject frequency range of the notch and thus help minimize excitation of these actuator vibrational modes. The technique utilizing notch filters allows the servo control system to effectively ignore lightly damped structural actuator resonances. At these resonances very little control is applied by the servo controller.
The drawback to this technique is that it depends on the ability of the designer to accurately predict the frequency of the vibrational modes. This becomes increasingly difficult in high accuracy regimes because the servo system is exposed to many unforeseen disturbances that excite unanticipated vibrational modes. For example, in a hard drive actuator such disturbances include servo amplifier saturation and distortion, external forces on the arm assembly, e.g., due to seek activity, air turbulence, stiction and the like. Such disturbances are typically generated at points in the control path where correction is impossible when gain stabilizing filters are present in the control loop. Consequently, although notch filters are useful in reducing predicted resonances of the servo control system, they do not inhibit the excitation of other vibrational modes by agents external to the servo control loop.
The prior art also teaches gain stabilization through low-pass filtering in the control loop. In this approach the cutoff frequency of a low-pass filter that is inserted in the control loop is generally lower than the frequencies of any of the lightly damped resonances of the actuator structure. Thus, the components of the control signal having the resonance frequency are effectively prevented from exciting the vibrational modes of the actuator structure. This helps ensure system stability, but it also increases the phase shift at frequencies in the vicinity of the servo loop's unity gain crossing, thereby reducing the bandwidth of the servo system. In fact, this drawback applies to all gain stabilizing filters, including notch filters. The reduction in bandwidth, in turn, reduces the ability of the servo system to correct low frequency vibration and tracking performance such as run out and other disturbances that are due to external excitation and non-linearities in positioning operations.
In alleviating the problem of stabilizing servo-controlled actuator systems, solutions using filtering techniques are inadequate in high-accuracy regimes, e.g., in high density hard disk drives, since they require a priori knowledge of the vibrational modes of the system. Meanwhile, solving a transfer function to determine the vibrational modes is computationally unfeasible or impossible in most practical cases. And, such prior art systems suffer from the limitation of not being able to actively compensate for multiple vibrational modes at the same time. Specifically, if more than one single mode is selected for active control system stability is at risk.
In the aforementioned U.S. Pat. No. 6,064,540, Huang, et al., suggest an active control mechanism and method for stabilizing a servo-controlled actuator system such as an actuator system in a data recording disk drive by compensating the vibrational modes of the actuator's arm assembly. The control mechanism has a sensing arrangement which can include one or more individual sensors attached to the actuator at locations where they generate signals in phase with the vibrational modes, and especially with all the major vibrational modes, of the arm assembly. A control mechanism derives from the signals an adjustment signal consisting of three corrective terms—a stiffening correction, an active damping correction and an inertia reduction correction—and the adjustment signal is used in the feedback control loop to stabilize the actuator system.
However, such conventional systems that use strain based sensors often need analog pre-amplifier sections to boost the signal from the sensor. Such amplification circuits consume electronic real-estate, and add to the costs of manufacturing and maintaining the disk drives.